1. Field of the Invention
The present invention relates to a friction material for brake to be used for brakes of vehicles or industrial machines, and more particularly to such a friction material excellent in frictional performance at high temperatures and mechanical strength without containing Cu based metals or antimony compound.
2. Description of the Related Art
With respect to friction materials, in non-asbestos based friction materials which have gradually become a main stream in substitution for existing asbestos based friction materials, there are employed many kinds of fibrous components, for example, for reinforcing strength, maintaining friction coefficient and improving qualities, and are much used metal fibers such as Cu fiber, steel fiber and the like.
Nowadays in general, the non-asbestos based friction materials contain about 0 to 20 vol % Cu fiber or Cu powder. Cu is useful for reinforcing strength, heightening friction coefficient of friction materials, and further maintaining friction coefficient at 400xc2x0 C. or higher and increasing heat radiating efficiency. Being different from steel fibers, Cu has characteristics less to have aggressiveness, which increases a wear amount of an opposite member such as a rotor, and to generate rusts.
Further, other than fibrous components, friction modifying components and binding components are contained therein, and such as Sb2S3 containing antimony of a heavy metal is served as friction modifying components.
Antimony compounds containing antimony of heavy metals as Sb2S3 or Sb2O3 are normally contained 0 to several vol % in the non-asbestos based friction material, and are useful for heightening lubrication at high temperatures and increasing noncombustibility of friction materials.
However, recently for attention around environmental sanitation, it has been demanded from mainly Europe and America to develop such friction materials without using heavy metal groups as said above.
Friction materials not containing heavy metals as Cu or antimony but having equivalent characteristics to those of the existing friction materials will be necessary hereafter, but as to Cu based materials, substituting materials therefor have not yet been found. In place of Cu based materials, there have conventionally existed semi-metallic friction materials using steel fibers having, however, the aggressiveness and being apt to causing rusts.
The invention is accordingly to solve problems as mentioned above, and it is an object of the invention to offer a friction material for brake without containing materials having heavy metals such as Cu or antimony. It is another object of the invention to offer a friction material for brake excellent in frictional performance at high temperatures and mechanical strength.
For accomplishing the objects of the invention, inventors of this invention made studies on materials substituting for Cu fibers, Cu powders, powders of Sb2S3 or Sb2O3. As a result, they found that the above mentioned objects can be accomplished by such a friction material containing magnesium oxide (MgO) and graphite at specific amounts and specific rates. Note that MgO has the low aggressiveness because of being comparatively soft among ceramic particle and high in heat conductivity, and graphite maintains friction coefficient at 400xc2x0 C. or lower and being useful for securing lubricity and heat conductivity.
That is, the present invention is concerned with a friction material containing fibrous components, binding components and friction modifying components, and is characterized in that magnesium oxide (Mgo) and graphite are contained in a friction material in the amount of 45 to 80 vol %, and volume ratio (MgO/graphite) of magnesium oxide (MgO) to graphite is 1/1 to 4/1.